Dr.Sum

There are tons of apps for brain exercise, but I can guarantee that this is the simplest but using your brain muscle effectively.

Rules for this game:

1. You should switch the numbers to make the sum of vertical/horizontal line for the listed answers each side.
2. Less you switch numbers to make all correct answers gain more scores of each turn.

Check out the video below.

I also love the game interface design so much. You have no need to watch any demos to play the game because it's so easy to understand the rule with few fingertips.